"Underground Coal Gasification (UCG) involves igniting coal in the ground, then collecting and using the gases that result from its partial combustion. Although the idea dates back over a century, very few UCG plants have ever been built. Underground gasification could potentially allow the use of coal that is currently uneconomical to mine. Underground gasification eliminates the need for strip mining and transportation of coal, as well as potentially making carbon capture and sequestration more practical. However, UCG produces large amounts of CO2, and the coal combustion wastes that are left behind can leach pollutants into nearby groundwater, and have caused major contamination in UCG pilot projects."[6]

"The form of UCG used in most modern projects consists of drilling two wells into a coal seam. Air or oxygen is injected into one well and a controlled combustion reaction is started in the seam itself - a more-controlled version of a natural coal seam fire. Gases are collected through the second well and are separated in a facility at the surface. This process produces primarily hydrogen and CO2, with lesser amounts of carbon monoxide, methane, and trace amounts of other gases."[6]

"These gases are then burned to produce energy, as in a natural gas plant. Hydrogen is the primary energy-containing gas in the mix. The combination of carbon monoxide and hydrogen is called syngas and can be combusted directly to produce heat or can be liquefied and refined in processes similar to that described for coal-to-liquids. Although CO2 is one of the major products of UCG, it is simply a waste product and does not contain any recoverable energy. Because the combustion occurs underground, it heats the surrounding rock. This portion of the heat is not accessible for industrial use, therefore UCG burns more coal (per unit energy produced) than would be required if it was first mined. However, conventional mining and transport of coal also requires significant energy and have associated releases of greenhouse gases."[6]

"Compared to conventional coal-fired power, underground gasification can greatly reduce the impacts associated with coal mining, coal dust, and the emissions of sulfur dioxide and nitrous oxides (responsible for acid rain and smog). However, UCG has several major environmental costs: contamination of ground water, subsidence of the overlying terrain, and global warming impacts. There is also a potential concern with the spread of underground coal seam fires, although this has not been observed in any trials to date."[6]

"In underground gasification, there is no need for above ground disposal of coal combustion wastes such as coal ash. However, these pollutants are left behind in the coal seam, where they can leach out into surrounding groundwater. Groundwater contamination has been a major problem in almost all pilot UCG projects, most most well-documented in the Hoe Creek project managed by Lawrence Livermore Laboratory. The combustion of coal traps a variety of toxic substances such as mercury, phenol, and benzene into the former coal seam. Additionally, the solubility of heavy metals in water can be increased if the coal seam is fully oxidized. Pollutants can leach out through the surrounding rock, or be taken up by water that enters the chamber. The Chinchilla pilot project attempted to address this problem by maintaining negative pressure inside the coal combustion chamber to limit the outflow of containments. However, the technology to prevent contamination is in its infancy, and groundwater pollution remains the single biggest concern with UCG. Groundwater contamination is particularly worrisome since the pollution source deep underground is inaccessible and permanent. Problems may crop up at any time during or after the project (including decades or centuries later) and cleanup will be difficult or impossible. Waste remaining underground presents a problem into the indefinite future, making eventual leakage likely."[6]

"Although the tectonic uplift of the 3000 m to 5000 m high range has generally been assumed to be mostly Miocene in age, field relationships suggest that the Domeyko Fault System and tectonic uplift were active as early as the Eocene, coinciding with porphyry copper emplacement between 41 Ma and 30 Ma. Apatite fission track (FT) thermochronology provides both age data and a time-temperature history for rocks since they cooled below a temperature of ca. 125 degrees C (equivalent to a depth of 4 km to 5 km under normal geothermal gradients) on their way to the surface during exhumation, or after a heating event."[7]

"Apatite FT data from the Paleozoic crystalline basement of the Domeyko Cordillera indicate that at least 4 km to 5 km of rocks were eroded during exhumation of this tectonic block between ca. 50 Ma to 30 Ma (Middle Eocene to Early Oligocene), a time that immediately precedes and overlaps with the emplacement of giant porphyry copper deposits. The FT data constrain the age and duration of a period of crustal thickening and extensive erosion known as the Incaic compression, an event recognized in the Andes of Chile and Peru."[7]